A descriptive study of the near-surface meteorology at three of the
potential Mars Exploration Rover (MER) landing sites (Terra Meridiani,
Gusev Crater, and Melas Chasma) is presented using global and mesoscale
models. The mesoscale model provides a detailed picture of meteorology
on scales down to a few kilometers but is not well constrained by
observations away from the Viking and Pathfinder landing sites. As such,
care must be taken in the interpretation of the results, with there
being high confidence that the types of circulations predicted will
indeed occur and somewhat less in the quantitative precision of the
predictions and the local-time phasing of predicted circulations. All
three landing sites are in the tropics and are affected by Hadley
circulation, by diurnal variations due to the global thermal tide, and
by planetary scale topography (in these particular cases from Tharsis,
Elysium, and the global topographic dichotomy boundary). Terra Meridiani
is least affected by large variations in local topography. Mean winds at
Terra Meridiani during MER landing would be less than 10 m/s with little
vertical shear. However, these low wind speeds result from strong mixing
in the early afternoon convective boundary layer, which creates its own
hazard in the horizontal variation of vertical winds of up to 8 m/s
(both upward and downward). In Gusev Crater the topography of Ma'adim
Vallis and the crater rim generates strong diurnally reversing
channeling of wind in Ma'adim Vallis and diurnally reversing radial flow
in the crater associated with thermal slope winds on the crater rim. The
overturning circulation in Gusev Crater slightly suppresses the daytime
convective boundary layer. Melas Chasma in Valles Marineris provides an
example of strong topographic forcing of near-surface circulation. Of
particular interest is the channeling of regional scale wind toward the
center of the Tharsis plateau during the evening. This results in a
surface level jet along the canyon of over 25 m/s. Drainage of air from
the plateau and into the canyon produces vertical winds down the canyon
walls in the evening of over 5 m/s. In contrast, during the early
afternoon (MER landing time), horizontal winds at the proposed MER
landing site are relatively calm, with little mean shear with height.
This results from the proposed site being in a region of local
divergence and the action of daytime convection. The nature of flow in
Melas Chasma results in an interesting dual maximum in boundary
convection and depth, with the usual daytime afternoon free convective
maximum being joined by a mechanically forced nighttime boundary layer
of almost 2 km depth.

Each Mars Exploration Rover (MER) is sensitive to the Martian winds encountered near the surface during the entry, descent, and landing (EDL) process. These winds are strongly influenced by local (mesoscale) conditions. In the absence of suitable wind observations, wind fields predicted by Martian mesoscale atmospheric models have been analyzed to guide landing site selection. In order to encompass the available models and render them useful to the EDL engineering team, a series of statistical techniques was applied to the model results. These analyses cover the high-priority landing sites during the expected landing times (1200-1500 LT). The number of sites studied is limited by the computational and analysis cost of the mesoscale models. The statistical measures concentrate on the effective mean wind (the wind as seen by the landing system) and on the vertical structure of the horizontal winds. Both aspects are potentially hazardous to the MER landing system. In addition, a number of individual wind profiles from the mesoscale model were processed into a form that can be used directly by the EDL Monte Carlo simulations. The statistical analysis indicates that the Meridiani Planum and Elysium landing sites are probably safe. The Gusev Crater and Isidis Basin sites may be safe, but further analysis by the EDL engineers will be necessary to quantify the actual risk. Finally, the winds at the Melas Chasma landing site (and presumably other Valles Marineris landing sites) are dangerous. While the statistical parameters selected for these studies were primarily of engineering and safety interest, the techniques are potentially useful for more general scientific analyses. One interesting result of the current analysis is that the depth of the convective boundary layer (and thus the resulting energy density) appears to be primarily driven by the existence of a well-organized mesoscale (or regional) circulation, primarily driven by large-scale topographic features at Mars.

The Mars Odyssey spacecraft entered into Martian orbit in October 2001
and after successful aerobraking, began mapping in February 2002.
Thermal infrared images taken by the Thermal Emission Imaging System
(THEMIS) on board the Odyssey spacecraft allow for the quantitative
retrieval of atmospheric dust and water ice aerosol optical depth. Data
collected so far cover late northern winter, spring, and summer
(Ls = 330°-160°). During this period, THEMIS observed
the decay of a regional dust storm, a number of local dust storms along
the edge of the retreating north polar cap, and the growth of the
low-latitude aphelion water ice cloud belt. Data from THEMIS complements
the concurrent Mars Global Surveyor Thermal Emission Spectrometer (TES)
data by sampling a later local time (~1400 LT for TES versus ~1600-1730
LT for THEMIS) and by observing at much higher spatial resolution.
Comparison of water ice optical depth in the aphelion cloud belt from
THEMIS and TES shows a significantly higher optical depth in the late
afternoon (THEMIS) than in the early afternoon (TES).

Intercomparison of thermal infrared data collected by Mariner 9, Viking,
and Mars Global Surveyor (MGS) is presented with a specific focus on air
temperatures, dust opacities, and water ice opacities. Emphasis is
placed on creating a uniform data set to most effectively reduce
interinstrument biases and offsets. The annual cycle consistently shows
a strong asymmetry about the equinoxes, with northern spring and summer
exhibiting relatively low temperatures, very high year-to-year
repeatability, and essentially no short-term (tens of days) variability.
The globally averaged Martian nighttime air temperatures close annually
to within a Kelvin during northern spring and summer. Daytime
temperatures show more variability (3-6 K). The difference in
repeatability of daytime versus nighttime temperatures is not
understood. Viking and MGS air temperatures are essentially
indistinguishable for this period, suggesting that the Viking and MGS
eras are characterized by essentially the same climatic state. Southern
summer is characterized by strong dust storm activity and hence strong
year-to-year air temperature variability. Dust opacity shows a
remarkable degree of interannual variability in southern spring and
summer, associated with the intermittent activity of regional and
planet-encircling dust storms, but exhibits high year-to-year
repeatability in northern spring and summer. Specifically, late northern
spring and early northern summer dust opacities appear to be completely
insensitive to the occurrence (or not) of major dust storms in the
previous southern spring or summer. We show that both Viking and MGS
data sets exhibit significant (and similar) polar cap edge dust storm
activity. The origins of the various major dust storms can be identified
in the thermal infrared data from Viking and MGS, including the
transport of dust from the northern autumn baroclinic zone into the
southern hemisphere tropics, which has also been identified in visible
imaging. We also note that the period around Ls = 225° is
characterized by very high dust opacities associated with dust storm
development or decay in every year thus far observed by spacecraft.
Water ice opacities have been retrieved from Viking infrared data for
the first time. We show that the northern spring and summer tropical
cloud belt structure and evolution are essentially the same in each of
the multiple years observed by Viking and MGS. Relatively subtle spatial
features recur in the cloud belt from year to year, suggesting the
influence of surface topography and thermophysical properties and a
reasonably consistent supply of water vapor. The seasonal evolution of
the tropical cloud belt through northern spring and summer is shown,
with the only significant deviations between years occurring from
Ls = 140° to 160°, where opacities fall in the second
MGS year associated with a small dust storm. Polar hood clouds are
observed in Viking and MGS observations with similar timing and extent.
Interactions between dust and water ice were highlighted in the Hellas
basin region during the southern spring 1977a and 2001 dust storms. The
observations demonstrate that the Martian atmosphere executes a very
``repeatable'' annual cycle of atmospheric phenomena. However, a major
part of this cycle is the occurrence of highly variable and potentially
major dust storm events. After such dust storm events the atmosphere
rapidly relaxes to its stable, repeatable state.

In this paper we define and describe morphological features that have
colloquially been termed "spiders" and map their distribution in the
south polar region of Mars. We show that these features go through a
distinct seasonal evolution, exhibiting dark plumes and associated
fan-shaped deposits during the local defrosting of the seasonal cap. We
have documented the seasonal evolution of the cryptic region and have
found that spiders only occur within this terrain. These observations
are consistent with a geyser-like model for spider formation.
Association with the transparent (cryptic) portion of the seasonal cap
is consistent with basal sublimation and the resulting venting of
CO2 gas. Also consistent with such venting is the observation
of dark fan-shaped deposits apparently emanating from spider centers.
Spiders are additionally confined to the polar layered deposits
presumably due to the poorly consolidated and easily eroded nature of
their upper surface.

An analysis of daily-to-interannual variability in the surface pressure
field of the Martian nothern hemisphere as given by a Martian climate
model is presented. In an empirical orthogonal function (EOF)
decomposition, the dominant first two modes of variability comprise a
zonal wavenumber 1 feature centered at 70 N latitude moving eastward
with a period of 6 to 8 sols. This feature is a baroclinic wave and
accounts for 53% of the northern hemisphere non-stationary surface
pressure variability, and, when active, has an amplitude of up to 2% of
local surface pressure. The third mode of the EOF decomposition is
annular about the Martian north pole, is null southward of 70 N, and
accounts for 7% of the northern hemisphere non-stationary surface
pressure variability. The baroclinic wave (EOFs 1 & 2) is active
during northern hemisphere winter and spring, consistent with models of
the Martian atmospheric circulation, and the annular mode (EOF 3) is
active only at the onset and demise of the baroclinic feature. When
active, it is not uncommon for the annular mode to reside in either its
positive or negative state stably for 20 to 30 sols. It is postulated
that baroclinic waves with longitudinal wavenumber 2, 3, and 4 act as a
pump for the annular mode. The annular mode should not be present in MGS
TES data.

The Thermal Emission Imaging System (THEMIS) on Mars Odyssey has
produced infrared to visible wavelength images of the martian surface
that show lithologically distinct layers with variable thickness,
implying temporal changes in the processes or environments during or
after their formation. Kilometer-scale exposures of bedrock are
observed; elsewhere airfall dust completely mantles the surface over
thousands of square kilometers. Mars has compositional variations at
100-meter scales, for example, an exposure of olivine-rich basalt in the
walls of Ganges Chasma. Thermally distinct ejecta facies occur around
some craters with variations associated with crater age. Polar
observations have identified temporal patches of water frost in the
north polar cap. No thermal signatures associated with endogenic heat
sources have been identified.

Variations in the Martian water and CO2 cycles with changes
in orbital and rotational parameters are examined using the Geophysical
Fluid Dynamics Laboratory Mars General Circulation Model. The model
allows for arbitrary specification of obliquity, eccentricity, and
argument of perihelion as well as the position and thickness of surface
ice. Exchange of CO2 between the surface and atmosphere is
modeled, generating seasonal cycles of surface ice and surface pressure.
Water is allowed to exchange between the surface and atmosphere, cloud
formation is treated, and both cloud and vapor are transported by
modeled winds and diffusion. Exchange of water and CO2 with
the subsurface is not allowed, and radiative effects of water vapor and
clouds are not treated. The seasonal cycle of CO2 is found to
become more extreme at high obliquity, as suggested by simple heat
balance models. Maximum pressures remain largely the same, but the
minima decrease substantially as more CO2 condenses in the
more extensive polar night. Vapor and cloud abundances increase
dramatically with obliquity. The stable location for surface ice moves
equatorward with increasing obliquity, such that by 45° obliquity,
water ice is stable in the tropics only. Ice is not spatially uniform,
but rather found preferentially in regions of high thermal inertia or
high topography. Eccentricity and argument of perihelion can provide a
second-order modification to the distribution of surface ice by altering
the temporal distribution of insolation at the poles. Further model
simulations reveal the robustness of these distributions for a variety
of initial conditions. Our findings shed light on the nature of
near-surface, ice-rich deposits at midlatitudes and low-latitudes on
Mars.

Large eddy simulations of vertical convective vortices and dust devils
in the Martian convective boundary layer are presented, employing a
version of the Mars MM5 mesoscale model, adapted to use periodic
boundary conditions and run at resolutions of 10 to 100 m. The effects
of background horizontal wind speed and shear on dust devil development
are studied in four simulations, each extending over the daytime portion
of one Martian day. The general vorticity development in all cases is
similar, with roughly equal positive and negative vorticity extrema. Two
dust devils were found to develop in the highest wind speed case and in
a case run without background wind. The dust devil structures were found
to agree well qualitatively with terrestrial dust devil observations,
including the prediction of greatly diminished vertical velocities in
the vortex core. Thermodynamic scaling theory of dust devils was found
to provide good prediction of the relationship between central pressure
and temperature in the modeled vortices. Examination of the turbulent
kinetic energy budgets suggests balance between buoyancy generation and
loss through dissipation and transport. The vorticity for the dust
devils is provided by twisting of horizontal vorticity into the
vertical. The horizontal vorticity originates from horizontal variations
in temperature at the lower boundary (thermal buoyancy). While the
horizontal winds generated by the modeled dust devils were likely
insufficient to lift dust, this study provides a solid starting point
for dynamic modeling of what may be an important component of the
Martian dust cycle.

We investigate the triggering mechanism of a cross-equatorial dust storm
observed by Mars Global Surveyor in 1999. This storm, which had a
significant impact on global mean temperatures, was seen in visible and
infrared data to commence with the transport of linear dust fronts from
the northern high latitudes into the southern tropics. However, other
similar transport events observed in northern fall and winter did not
lead to large dust storms. Based on off-line Lagrangian particle
transport analysis using a high resolution Mars general circulation
model, we propose a simple explanation for the diurnal, seasonal and
interannual variability of this type of frontal activity, and of the
resulting dust storms, that highlights the cooperative interaction
between northern hemisphere fronts associated with low pressure cyclones
and tidally-modified return branch of the Hadley circulation.

We present the first comprehensive general circulation model study of
water ice condensation and cloud formation in the Martian atmosphere. We
focus on the effects of condensation in limiting the vertical
distribution and transport of water and on the importance of
condensation for the generation of the observed Martian water cycle. We
do not treat cloud ice radiative effects, ice sedimentation rates are
prescribed, and we do not treat interactions between dust and cloud ice.
The model generates cloud in a manner consistent with earlier
one-dimensional (1-D) model results, typically evolving a uniform
(constant mass mixing ratio) vertical distribution of vapor, which is
capped by cloud at the level where the condensation point temperature is
reached. Because of this vertical distribution of water, the Martian
atmosphere is generally very far from fully saturated, in contrast to
suggestions based upon interpretation of Viking data. This discrepancy
results from inaccurate representation of the diurnal cycle of air
temperatures in the Viking Infrared Thermal Mapper (IRTM) data. In fact,
the model suggests that only the northern polar atmosphere in summer is
consistently near its column-integrated holding capacity. In this case,
the column amount is determined primarily by the temperature of the
northern polar ice cap. Comparison of the water cycle generated by the
model with and without atmospheric ice condensation and precipitation
shows two major roles for water ice cloud. First, clouds are essential
to the observed rapid return of atmospheric water to the surface in late
northern summer, as ice sedimentation forces the water column to shrink
in response to the downward motion of the condensation level,
concentrating water near surface sinks. Second, ice sedimentation limits
the amount of water that is transported between the hemispheres through
the Hadley circulation. This latter effect is used to greatly improve
the model simulation of the annual water cycle by increasing ice
sedimentation rates. The model is thus shown to be able to reasonably
reproduce the annual cycles of vapor and ice cloud as compared to Viking
data. In addition, the model is shown able to reproduce
near-instantaneous maps of water ice derived from Hubble Space Telescope
images. The seasonal evolution of the geographic distribution of water
ice compares reasonably well with Viking and Mars Global Surveyor (MGS)
Mars Orbiter Laser Altimeter (MOLA) observations, except in the
prediction of a weak tropical cloud belt in southern summer. Finally, it
is shown that the tropical cloud belt is generated in the model by the
cooling of water vapor entrained in the upwelling branch of the Hadley
cell. Decline of the tropical cloud belt in mid northern summer is shown
to be related to an increase in air temperatures, rather than to
decreases in water vapor supply or the vigor of Hadley cell ascent. By
equinox, the cloud belt experiences a second major decline event, this
time due to a reduction in vapor supply. The ability of the model to
emulate many aspects of observed cloud behavior is encouraging, as is
the ability of enhanced ice sedimentation to improve the overall quality
of the water cycle simulation. However, significant work remains to be
done before all observational constraints can be matched simultaneously.
Specifically, in order for the generally good fit to all other data to
be attained, cloud ice particle sizes about an order of magnitude too
large must be used.

Surface wind stresses and dust lifting in the south polar region of Mars
are examined with a three-dimensional numerical model. The focus of this
study is the middle to late southern spring period when cap-edge dust
lifting events are observed. Mesoscale model simulations of high
southern latitudes are conducted at three dates within this season
(Ls = 225°, 255°, and 310°). Assuming that dust
injection is related to the saltation of sand-sized grains or
aggregates, the Mars MM5 mesoscale model predicts surface wind stresses
of sufficient strength to initiate movement of sand-sized particles
(~100 μm), and hence dust lifting, during all three periods. The
availability of dust and/or sand-sized particles is not addressed within
this study. Instead, the degree to which the existence of sufficiently
strong winds limit dust injection is examined. By eliminating forcing
elements from the model, the important dynamical modes generating high
wind stresses are isolated. The direct cap-edge thermal contrast (and
topographic slopes in some locations) provides the primary drive for
high surface wind stresses at the cap edge, while sublimation flow is
not found to be particularly important, at these three dates.
Simulations in which dust is injected into the lowest model layer when
wind stresses exceed a threshold show similar patterns of atmospheric
dust to those seen in recent observations. Comparison between these
simulations and those without active dust injection shows no signs of
consistent positive or negative feedback due to dust clouds on the
surface wind stress fields during the late spring season examined here.

The Pennsylvania State University/National Center for Atmosphere
Research Mesoscale Model Version 5 (MM5) has been converted for use on
Mars. Modifications are based on schemes implemented in the Geophysical
Fluid Dynamics Laboratory Mars General Circulation Model (GCM).
Validation of the Mars MM5 is conducted by comparison to the Mars GCM,
examining the large-scale dynamics in the two models. Agreement between
the two models on similar scales (a few hundred kilometers) is good.
Validation is also performed against both Viking Landers and Mars
Pathfinder meteorological observations with the model run at higher
vertical (lowest level at 1.6 m) and horizontal resolution (a few
kilometers). We find reasonable agreement with near-surface air
temperature, pressure, and wind direction observations, with caveats.
The results demonstrate that the model accurately simulates surface heat
balance and the propagation of global thermal tides. However, wind
speeds are underpredicted. The model generates the correct phasing of
wind speeds with local time at the Viking Lander 2 site during winter
but does not generate the correct phasing at the other sites or seasons.
We examined the importance of slopes and global tides in generating the
diurnal cycle of winds at the lander sites. We find that tides are at
least as important as slopes, in contrast to previous studies. This
study suggests that when used in combination with a GCM, the Mars MM5
promises to be a powerful tool for the investigation of processes
central to the Martian climate on scales from hundreds of kilometers to
tens of meters.

We describe the first use of a general circulation model to study the
Martian water cycle. Water is treated as a passive tracer, except for
ice-albedo coupling. The model is used to assess which mechanisms and
water reservoirs are critical to the seasonal evolution of water and
specifically the attainment of an interannually repeatable steady state.
The model comes to a reasonable steady state with active surface ice and
atmospheric vapor and ice reservoirs. A regolith is not necessary. The
mechanism of equilibration results from independent parameters
controlling the transport of water between the northern polar and the
extratropical atmospheres at different seasons. Water export from the
northern summer pole results from weak mixing across a strong vapor
gradient, dependent upon northern cap temperatures. Import at other
seasons depends on stronger mixing and weak vapor gradients, which are
history dependent. Equilibration is achieved when the fluxes balance,
minus a small net loss to the south. We find that with a southern
residual CO2 cap, the water cycle cannot be completely
closed. We conclude that the northern summer cap temperature determines
the bulk humidity of the atmosphere, all else being equal. We proceed to
show that a water cap exposed in southern summer would be unstable with
respect to the north for dynamical as well as thermal reasons. At high
obliquity (45°), much higher vapor abundances result in more
widespread surface ice with seasonal ice caps overlapping in the
equinoctial subtropics, producing year-round stability of water ice just
north of the equator.

Large seasonal and hemispheric asymmetries in the martian climate system
are generally ascribed to variations in solar heating associated with
orbital eccentricity. As the orbital elements slowly change (over a
period of >104 years), characteristics of the climate such
as dustiness and the vigour of atmospheric circulation are thought to
vary, as should asymmetries in the climate (for example, the deposition
of water ice at the northern versus the southern pole). Such orbitally
driven climate change might be responsible for the observed layering in
Mars' polar deposits by modulating deposition of dust and water ice.
Most current theories assume that climate asymmetries completely reverse
as the angular distance between equinox and perihelion changes by
180°. Here we describe a major climate mechanism that will not
precess in this way. We show that Mars' global north-south elevation
difference forces a dominant southern summer Hadley circulation that is
independent of perihelion timing. The Hadley circulation, a tropical
overturning cell responsible for trade winds, largely controls
interhemispheric transport of water and the bulk dustiness of the
atmosphere. The topography therefore imprints a strong handedness on
climate, with water ice and the active formation of polar layered
deposits more likely in the north.